There is, by now, a familiar script to it all: A mass public shooting, followed by waves of grief and outrage, then calls for gun control on the one hand and harrumphing about politicizing tragedies on the other. The news stories and statements by political leaders write themselves, with only the location, name of the shooter and number of casualties changing. It all seems so routine.

But this perception isn’t because of some unprecedented rise in the rate of mass public shootings—far from it. They’re roughly as common now as they were in the 1980s and ’90s. And the data offer a stark finding: Over the past decade, mass public shootings haven’t become particularly more prevalent, they’ve simply become deadlier.

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A mass public shooting, as I’ve defined it in my research, is any incident in which four or more victims are killed with a firearm within a 24-hour period at a public location in the absence of other criminal activity (robberies, drug deals, gang “turf wars”), military conflict or collective violence. For instance, last year’s mass murder at the Pulse nightclub in Orlando would qualify, but the 1890 massacre at Wounded Knee would not, nor would a familicide in a private home, nor a shootout between rival gangs.

Within these boundaries, there have been 140 mass public shootings in the United States since 1976 (the rest have been mostly familicides and felony-related massacres), which amounts to an average of a little more than three per year. When we’re looking at trends in the incidence and severity of mass public shootings over time, it’s necessary—just as it is with other types of crime—to adjust for changes in the size of the U.S. population.

For example, we had roughly the same raw number of murders in the U.S. in 2011 (14,612) as in 1969 (14,760). But because there were 110 million more Americans in 2011, the 1969 murder rate per 100,000 residents was 7.3, about 55 percent higher than the 2011 rate (4.7). Due to the rarity of mass public shootings, I’ve calculated the rates per 100 million in the U.S. population. (And, to help clarify the direction of trends over time, the black lines in the graphs below represent a five-year moving average.)

Mass Public Shooting Rate (per 100 million) from 1976-2016

Grant Duwe

Since the mid-2000s, the incidence of mass public shootings on a per capita basis has been a bit higher than it was in the preceding 10 years. But the rates over the past 10 years are no higher than in the late 1980s and early '90s, when the frequency of mass public shootings led to the creation of policies designed to address violence in schools and workplaces. Most notably, the growing number of high-profile mass public shootings in that era helped bring about the 1994 enactment of the federal assault weapons ban, which was allowed to expire in 2004.

What has increased over time is the number of people shot in these incidents. Looking at annual trends in the total number of victims shot in mass public shootings (on a per capita basis), you can see that the severity has recently increased, reaching a 40-year high. Because the trends in the rates at which victims have been killed and wounded have been similar, I focus on the total number of victims shot (either killed or wounded). Before 2012, the five-year moving average never exceeded 20 victims shot (per 100 million Americans). Since then, the five-year moving average rate has been above 20 every year but one (2014).

Total Victims in Mass Public Shootings (per 100 million) from 1976-2016

Grant Duwe

This may help to explain why shootings seem more common, even though they aren’t. Research shows that the number of victims killed and wounded are the strongest predictors of the extent to which a mass killing gets reported by the news media. Recent growth in the number of catastrophic mass public shootings—combined with the extensive, wall-to-wall news coverage that accompanies these tragedies—likely accounts for the commonly held misconception that mass shootings are now more frequent.

The rise in the average number of victims also raises a number of other questions about mass public shootings. Foremost among them: Why have they become more deadly since the mid-2000s?

It may be tempting to conclude this increase is because of the expiration of the assault weapons ban in 2004—after all, the increase began shortly after the ban ended. But the limited research that’s been done suggests it had little short-term impact on gun violence.

That’s probably not a popular conclusion. But the available evidence suggests that strengthening or weakening gun laws would not significantly affect the incidence or severity of mass public shootings. For example, studies examining bans on large-capacity magazines and right-to-carry concealed firearms laws have found they would have little or no effect on mass public shootings.

Still, the question of whether the assault weapons ban had an impact on the severity of mass public shootings has yet to be answered empirically, which highlights a surprising major problem for those of us who’d like to stop the killings: There’s been relatively little rigorous research on mass violence, likely due to the virtual absence of research funding on this topic. In comparison, we spend millions each year to fund research on tornadoes, which have been about as deadly as mass shootings since the mid-1970s.

The few studies we do have tell us that mass public shootings, while horrific, are, fortunately, quite rare. This apparent paradox—rare yet “routine”—likely reflects the outsized impact that catastrophic mass murders have on our perceptions of public safety. But until we make the investment to find solutions, we won’t really know why these tragedies happen or how to prevent them.

Grant Duwe is research director for the Minnesota Department of Corrections and author of Mass Murder in the United States: A History. The ideas expressed here represent the views of the author and not necessarily those of the Minnesota Department of Corrections.